Mass may be celebrated only on a consecrated fixed or portable altar. A fixed altar, a permanent structure of stone, must be consecrated by the bishop of the diocese, or by another bishop with his consent. Fixed altars are consecrated in consecrated churches, or in those which, though only solemnly blessed, will later be consecrated. The ceremony consists in placing in the stone-covered cavity in the center of the altar-table, near the front edge, relics of two canonizedmartyrs and three grains of incense, anointing the altar with holy oils, incensing it, burning incense upon it, and celebrating Mass upon it. When a number of portable altar-stones are consecrated, Mass is said on one of them.